a. Field of the Invention
This invention pertains to improvements in slurry borehole mining and more particularly relates to a drilling and mining method and apparatus for recovering deep subterranean deposits of ore with a dual conduit drill pipe wherein the drilling and mining fluid is directed into the annular section between the conduits and the drill cuttings and slurry are returned to the surface in the inner conduit; a hydraulic jet pump located intermediate of the mining tool section and the slurry discharge is used to pump the ore slurry to the surface. A hydraulic jet pump located at the base of the tool pumps drill cuttings during the drilling mode through the inner conduit to the surface.
b. Description of the Prior Art
Subterranean ore deposits have been discovered that cannot be mined by conventional mining apparatus or methods, particularly where a water table is encountered. Various methods have been used in attempts to recover granular ore deposits at depths of 150 to 300 ft.; prior art methods employing a hydraulic jet pump for the ore slurry have had a practical depth limit of 500 ft. The methods and apparatus used in these experiments resulted in subsidence of large quantities of overburden into the initial ore cavity and with the tool pumping the overburden to the surface.
Hydraulic borehole mining of ore is broadly known in the art as evidenced by the following United States Patents:
U.S. Pat. Nos. 3,155,177 and 3,316,985 issued to A. B. Fly on Nov. 3, 1964 and May 2, 1967 respectively teach a method and apparatus for slurry mining through a borehole which may be changed between its drilling mode of operation and its mining mode of operation to mine several different stratas without removing the tool string from the borehole. The drill string is equipped with a conventional drill bit enabling the machine to drill through hard formations; the drill bit being driven by a hydraulic drilling device. The gross suspended weight of the drill string or any portion thereof can be applied to the roller bit to facilitate optimum drilling rates. The invention teaches the essential controls needed to convert from drilling to mining, i.e., mining nozzle flow, eductor nozzle flow and fluid flow to the drill bit each with a drilling mode and a mining mode of operation. Electric motors within the tool string operate the valves to convert the apparatus from the drilling mode to the mining mode of operation while the tool is still in the borehole. The inventor anticipates jetting the matrix in air; the fluid level in the bore or well cavity being maintained below the mining jets and above the eductor pump opening for optimum operation. The hydraulic eductor located below the mining nozzle is subject to cavitation at the extreme venturi velocities that are needed for maximum pumping depths and acts to limit the practical depth (500 Ft.) to which the tool can be used.
U.S. Pat. No. 2,518,591 which issued to Aston et al on Aug. 15, 1950 discloses a jet mining and excavating apparatus wherein water jets are used to sink boreholes into alluvial soils and the inventor expressly anticipates the use of a combined sinking and excavating unit and additionally anticipates the use of the usual boring means instead of washing to produce boreholes in hard ground or rock extending to the mineralized zone. The apparatus employes a water jet to slurrify the ore and a eductor to raise the slurry to the surface. When the borehole reaches the mineralized stratum, the sinking unit is removed and an excavating unit substituted therefore.
U.S. Pat. Nos. 3,730,592 issued May 1, 1973 and 3,747,696 issued July 24, 1973 to Wenneborg et al discloses a method and apparatus for borehole mining wherein the drilling and mining fluid is pumped from the surface through an annulus in the tool string and two distinct paths are employed to return the fluid to the surface; while drilling the fluid is returned externally of the tool through the space between the tool and the wellbore to `wash` the cuttings to the surface, during mining fluid is directed into the orebody to form a slurry and the slurry is pumped to the surface within the tool by an eductor located above the foot valve and adjacent to the slurry entrance to the tool. The mining nozzle is located above the diffuser section of the eductor and attacks the orebody at the top of the strata. An important feature of this disclosure is that the hydraulic actuating force for changing the function of the tool from drilling to mining is transmitted from the surface and that the actuating fluid line is located wholly within the fluid supply passage to the lower tool section; the hydraulic valve operating means is used to operate the mining nozzle, foot valve and a slurry eductor located at the base of the tool at the slurry entrance.
U.S. Pat. No. 4,035,023 filed July 15, 1975 and issued July 12, 1977 to Clifford Cockrell discloses the use of a rotatable concentric dual-conduit tool string with Hydril type screwed joints wherein the first fluid is directed down the annular space between the conduits and the slurry is returned to the surface through the inner conduit. The cavity is subjected to air pressure sufficient to raise the slurry to the surface via the slurry discharge conduit. A foot valve controls the volume of slurry rising to the surface and is hydraulically operated by the differential pressure between the vented cavity pressure at the base of the tool and the hydraulic pressure supplied by a slurry level controller.
In a modified embodiment of the invention a turbine driven multi-stage slurry pump is used to pump the slurry to the surface. An auxiliary air lift means can be used to assist lifting the slurry to the ground level.
In this disclosure Cockrell is the first to teach a principle of self-activating down-hole hydraulic valves for tool control; he is first to disclose, describe and use the differential pressure between a control pressure equivalent to or less than system pressure and the vented cavity pressure to establish a hydraulic actuating force for down-hole hydraulic valve control. Cockrell is also first to teach, describe and use the differential hydraulic actuating force between a control pressure and the cavity pressure to modulate down-hole control valves.
U.S. Pat. No. 4,059,166 of Nov. 22, 1977 and No. 4,067,617 of Jan. 10, 1978 issued to P. R. Bunnelle disclose a method and apparatus for slurry mining with one or more mining nozzles disposed above the eductor. Two distinct paths are used to return the fluid pumped to the tool in the well bore; during drilling the fluid and cuttings are returned externally of the tool string through the space between the tool and the well bore, while during mining the ore slurry is returned within the tool. Several different hydraulic control systems are disclosed to convert the down-hole tool from drilling to mining all with valve control means located within the fluid supply passage of the tool. A separate drilling head and mining head are used in this disclosure. A control system employing a hydraulic conduit extending to the surface is used to modulate the eductor nozzle to control the cavity pressure; control systems activated by the difference between the pump pressure and a vented or cavity pressure are also employed. Self-activating systems to modulate the eductor nozzle also employ the pressure difference between a control pressure and a vented cavity pressure for valve actuation. Separate control systems are employed for the eductor and the foot valve. An important feature of these systems is that the valve actuating control pressure and the drilling pressure are the same during drilling while during mining the valve actuating control pressure is established by venting to the cavity pressure or to the atmosphere.
U.S. Pat. No. 4,077,671 of Mar. 7, 1978 and No. 4,077,481 of Mar. 7, 1978 issued to P. R. Bunnelle disclose a method and apparatus for a combination drilling and mining tool wherein the drilling water is directed through the outer annular conduit and through a tool bit foot valve to flush the cuttings upwardly to the surface externally of the tool between the drill string and the borehole; after drilling a mining head replaces the drilling head on the upper end of the drill string which also provides hydraulic control means for changing the tool function from drilling to mining and for certain modes of tool operation. When the ore is recovered from one elevation and it is desired to remove ore from a deeper strata, the mining head is removed and replaced by a drilling head, additional pipe sections being added until the lower strata is encountered; the drill head must then be removed and replaced by a mining head before ore recovery is initiated. In another embodiment two mining nozzles are assembled into the tool string each at different elevations to permit mining two strata of ore from different levels. Hydraulic valve control systems are located wholly within the fluid supply passage of the tool.
A modified valve control system is provided which is self-activating by responding to the pressure differential between the system pressure and the pressure in the well cavity. An important feature of this method is that during the drilling mode the valve actuating control pressure and the drilling pressure are the same while during the mining mode the valve actuating control pressure is established by venting to the atmosphere or to the cavity pressure.
The practical application of this tool is limited to recovering ore deposits at depths up to 500 Ft. based on its ability to pump commercial quantities of ore to the surface.
Other patents relating to apparatus or methods for mining slurrified ore but appearing to be less pertinent include: U.S. Pat. Nos. 3,311,414; 3,439,953; 4,140,346; 3,623,558; 3,797,590; 3,030,086 and 4,134,619.